Wildlife are a serious economic problem in certain agricultural, industrial, commercial and urban environments. Deer, moose, and elk browse on tree seedlings planted by foresters (Black, 1992), and Feed on a variety of ornamental plants. Beaver cut trees and flood out others. Coyotes and wolves kill grazing sheep (Gaafar et al., 1985). Rabbits and hares are a constant problem in newly planted fields and reforested areas. Gophers chew through underground communications cables requiring extensive searches for the damaged section and subsequently expensive repairs. Birds feed on seed sown for agricultural purposes. Pigeons deface commercial properties. There is a continuing need to control these wild species because of the extensive economical damage they cause (Marsh 1986, 1988, 1990). Products which when ingested by wildlife cause nausea, agitation, malaise and gastrointestinal illness are potential aversion conditioning agents. The effect must be fast so that the animal associates the feeling with the ingested target item and this association must be long lasting, i.e. remembered and related to the target.
Wildlife naturally learn to avoid certain plants for reasons which include taste, odor and illness. Plant-derived products are especially attractive as wildlife repellent agents because they are already in the environment, protecting these species against predators. Natural products which have been investigated as animal repellents include bitter quinine, hot tasting capsaicin from peppers, bone tar oil and putrescent whole egg solids. Other plant derived products considered as potential animal repellents include simmondsin in jojoba seeds, strychnine, red squill powders and digitalis glycosides. Digitalis extracts were originally bioassayed in pigeons for emesis as an estimate of potency. The digitalis glycosides cause nausea and vomiting as a side effect to their normal cardiotonic activity. Digitalis lanata and D. purpurea are cultivated in the Netherlands. Red Squill, Urginea maritima, contains the toxic and emetic glycoside scilliroside. Scilliroside and other red squill preparations have been registered in the United States as rodenticicles, but supplies of this slow growing Mediterranean region bulb plant are limited. Scilliroside emesis occurs at lower doses than its lethality in humans and other animal species, thereby serving as a safety factor.
Nerium oleander is an evergreen plant that is indigenous to the Mediterranean region, sub-tropical Asia, and India. It is now ubiquitous California, Arizona, Florida, and other warmer climate states. It is a common ornamental plant used in urban gardens, along highways, golf courses and other municipal facility landscapes. The plant grows well with minimal water and is universally avoided by wildlife, probably because of its emetic activity. In Asia oleander is considered a therapeutic plant used for treating swelling, leprosy, eye and skin disorders. The leaves also possess diuretic and cardiotonic properties, principally due to the presence of oleandrin, a digitalis-like glycoside. Oleandrin occurs in leaves, stems, floral parts and resin. Oleander also contains oleandrin related glucoside conjugates. Oleandrin itself and these glucosides yield the aglycone oleandrigenin on acid hydrolysis. The glycoside conjugate of oleandrin is oleandrose. The active components of oleandrin and oleandrigenin are tasteless, odorless and highly emetic.
The digitalis-derived aglycones gitoxigenin, digitoxigenin and digoxigenin have similar structures. Scillirosidin, oleandrin and oleandrigenin differ from the digitalis aglycones in that they contain acetoxy groups which increases their activity. It is important that there is a dose differential between the toxic and emetic activity in these plant derived products. The products will all degrade in the environment due to hydrolysis of their acetoxy and lactone groups, rendering them inactive.